We are perhaps more accustomed to praying to God as active and creative than as resting and celebrating, but it would be a mistake for us to make too strong an ontological connection between God's creativity and God's being. If God's being consists of activity and creativity, then we are left with a very busy God for whom sabbath rest must be a departure from God's very being rather than a retreat thereto... 'Retreat' probably isn't the right word actually... but sabbath rest, as the grand finale and the final goal of the creative process, has a definite eschatological quality and, as such, it has a certain ontological quality as well. Because sabbath gives us a picture of the goal, the end of God's work, it thus tells us who God is and what constitutes God's being. Since God corresponds to God's self-revelation, what God will do must correspond to who God is and thus God is who God is becoming, who God is in the end is what constitutes God's being in the present. And God's work of salvation in the present is in the service of an end, a Sabbath, in which God and creation may celebrate and rest in relation to one another. Therefore the work itself, though it has correspondence in God's being insofar as it is the working-out of the relationship which God has chosen with creation, does not determine who God is. God is still God even when the work is done.
Downshift a little for this... Since God's rest presupposes God's relationship with creation, God's being is not static. It assumes that which it saves. The experience of creation itself is assumed into the divine. Humanity, in all its frailty, is not banished from the garden. God's work of salvation in Christ assumes humanity into God's being so that God's celebration and rest includes the redemption of humanity. So while the work itself does not determine who God is, the object of God's work--the object of God's love--is taken freely into the life and being of God. Saving work corresponds to God insofar as the object of saving work corresponds to God in God's communion therewith.
If work, even if it's saving or creating work, is too tightly connected to the being of God then we are bound to relate to God not as a being-in-relationship but as a function or an object. So in order to relate to God on God's own terms, when we pray to the creative God or the saving God, we must presuppose that we are praying to a resting God, a celebrating God, a God who has taken our death and humanity into God's being and has finished the work of salvation. Sabbath rest is revelation of God. When by grace we receive the Sabbath of God, we receive God and we receive ourselves as beings-in-relationship, free from possession and function, free for God and free for neighbor. Sabbath ontologically liberates the world to love.